This invention relates to a vascular biomaterial, particularly to the use of expanded polytetrafluoroethylene in a vascular biomaterial.
Autologous saphenous vein is the conduit of choice for replacement of small diameter, diseased arteries in cardiac and vascular surgery. However, in certain cases, it is unsuitable for use and expanded polytetrafluoroethylene (PTFE) has served as the best synthetic alternative.
Expanded PTFE has a delicate microfibrillar, porous structure. Typically, the microfibrils have a cross-sectional diameter of less than about 1 micrometer. Long-term applications of an expanded PTFE graft as an arterial substitute, are limited by a high incidence of occlusion by thrombus as a result of incompatibility with blood. Lack of good tissue ingrowth is another problem.
As illustrated by U.S. Pat. No. 3,635,938 to Ryan et al, and by Roberts, Ryan et al, Journal of Applied Polymer Science, 20: 255 (1976), it is known to bond PTFE to an adhesive and to increase gold-Teflon FEP joint strength. In the Ryan patent, activated aluminum foil is used to form an intermediate PTFE-aluminum composite, and the aluminum foil is dissolved in a sodium hydroxide solution.
In the Roberts publication, an aluminum layer is deposited onto Teflon FEP by evaporation, the aluminum layer is removed with a sodium hydroxide solution, and a gold-Teflon FEP composite is formed. Electric properties of the virgin polymer material were unaffected, and XPS analysis was concluded to show oxygen-containing hydrocarbon species in the surface region of a chemically-modified Teflon FEP.
Also known is the modification of surface chemistry of PTFE by the introduction of oxygenated moieties, as exemplified by Costello and McCarthy, MacromolecuIes, 20: 2819 (1987), Morra et al, Langmuir, 5: 872 (1989), and the work of J. A. Gardella, Jr. and T. G. Vargo at the University at Buffalo, S.U.N.Y., Buffalo, N.Y. The Costello process creates surface-residing hydroxyl groups by a reduction step followed by hydroboration and an oxidation step. However, a drawback is the depth to which the reduction step etches into the bulk of the PTFE, with a depth of about 150 to 20,000 Angstroms being cited. Expanded PTFE is said to react with the reducing agent.
The Morra et al procedure utilizes oxygen plasma to treat PTFE. The University at Buffalo work utilizes RFGD plasmas comprising water or methanol vapors mixed with hydrogen gas to hydroxylate expanded PTFE, and describes the preparation of silanized expanded PTFE by treatment with (3-aminopropyl)triethoxysilane, and subsequent derivation with FITC; however, such work changes underlying microfibrils and has not controllably provided for significantly varied degrees of hydroxlation.
The design of polymers which mimic the haemocompatible surface of the endothelial cell wall, is exemplified by Hayward and Chapman, Biomaterials, 5: 135 (1984). This prior art describes a haemocompatible membrane formed of phospholipid polymers.
As illustrated by Albrecht et al, Biochimica et Biophysica Acta. 687: 165 (1982), phospholipid membranes have been immobilized onto PTFE surfaces using Langmuir-Blodgett monolayer deposition followed by cross-linking polymerization of the membrane. Other work lacking covalent bonding between a polymer surface and a haemocompatible compound is exemplified by Chemical Abstracts, 102(14): 119697x (1984), which plasma treats a polymer surface and heparinizes the resultant surface after an intermediate surfactant exposure step.
Covalent bonding of haemocompatible compounds to various substrates other than PTFE is known, as exemplified by Jozefowicz and Jozefowicz, Pure & Appl. Chem., 56(10): 1335 (1984), Chemical Abstracts, 105(6): 49107r (1986), and U.S. Pat. No. 4,824,529 to Thompson et al. The Chemical Abstracts publication discloses a silane coupling agent, and Example 5 of the Thompson patent describes the use of (3-aminopropyl)triethoxysilane as a coupling agent. Covalent bonding produces a stabilized biomaterial.
Therefore, there is a need for an expanded PTFE to which a haemocompatible membrane layer has been covalently bonded, for use in a vascular biomaterial. Importantly, the expanded PTFE would continue to possess adequate physical properties to ensure safe clinical use. Additionally, it would be advantageous if good tissue ingrowth were promoted. Likewise, a process for making such a vascular biomaterial is needed.